This invention generally relates to a lasing system employing a wavelength-conversion waveguide within which semiconductor laser light is propagated while at the same time being converted into second-harmonic light. This invention has significant applications in optical recording/reproducing, laser printing, and laser instrumentation.
As the density of optical disks and the quality of laser printing have become higher, semiconductor laser light sources capable of providing a shorter-wavelength output have also been required. Semiconductor laser light sources now in use are capable of emitting red light having a wavelength of up to about 630 nm, and the semiconductor laser industry is now trying to provide an improved semiconductor laser light source capable of producing light (green, blue, and ultraviolet) having a much shorter wavelength.
A short-wavelength lasing system using a semiconductor laser has been proposed. In this system, semiconductor laser light in the near-infrared region emerging from the semiconductor laser is converted into second-harmonic light for emission of blue or ultraviolet laser light.
When generating second harmonic light from a non-linear optical material at high efficiency, it is required to equalize the propagation constant of a fundamental with the propagation constant of a second harmonic. To achieve this generation requirement, N.sub..omega. (i.e., the refractive index to the fundamental) must be equal to N.sub.2.omega. (the refractive index to the second harmonic). However, for the case of bulk materials, N.sub.2.omega. is usually greater than N.sub..omega. i.e., N.sub.2.omega. &gt;N.sub..omega., due to the wavelength dispersion of refractive index. This does not meet the aforesaid second-harmonic light generation requirement.
Where a wavelength-conversion waveguide is used, light travelling within the wavelength-conversion waveguide goes into a native mode having a specified propagation constant. Such a propagation constant depends on the size of wave-length-conversion waveguides. From this fact, a requirement that two times the fundamental propagation constant (.beta..sub..omega.) is equal to the second-harmonic propagation constant (.beta..sub.2.omega.) can be met by changing the width and depth of a wavelength-conversion waveguide. When making use of a wavelength-conversion waveguide, greater fundamental/second-harmonic superposition provides higher conversion efficiency and higher fundamental power density and also provides higher conversion efficiency. Therefore, lower modes are preferable.
Recently, a technique has attracted attention in which semiconductor laser light is coupled to a wavelength-conversion waveguide formed on a substrate having a great non-linear optical constant (e.g., a LiNbO.sub.3 substrate, a LiTaO.sub.3 substrate, and a KTiOPO.sub.4 (KTP) substrate) for conversion into second-harmonic light, to accomplish high conversion efficiency.
Referring to FIG. 13, a conventional lasing system employing a wavelength-conversion waveguide will now be described below.
As shown in FIG. 13, semiconductor laser light having a wavelength of 860 nm emitted from a semiconductor laser 50 passes through a collimating lens 51 and a focusing lens 52 and is coupled to an incident portion of a wavelength-conversion waveguide 54 formed in a LiTaO.sub.3 substrate 53 having a z-axis facet. This 860 nm-wavelength semiconductor laser light is propagated through the waveguide 54 while at the same time being converted into second-harmonic light having a wavelength of 430 nm by means of a polarization inversion region 55 formed in the LiTaO.sub.3 substrate 53.
Both the waveguide 54 and the polarization inversion region 55 are formed on the LiTaO.sub.3 substrate 53 by means of proton exchange, and the period of the polarization inversion region 55 is determined by the wavelength of semiconductor laser light to be converted into second-harmonic light.
The period of the polarization inversion region 55 formed by means of proton exchange usually deviates from design values. Therefore, a technique to modulate the wavelength of semiconductor laser light is employed to achieve high conversion efficiency. Since the wavelength of semiconductor laser light is of temperature- and output-dependency, a mechanism capable of performing a certain modulation in wavelength is required to obtain high conversion efficiency.
A collimating lens 56 makes the second-harmonic light from the waveguide 54 parallel and the collimated light is emitted from an output mirror 57. In FIG. 13, however, diffraction/reflection light from a diffraction grating 58 is fed back to the semiconductor laser 50 for modulation in wavelength. By modulating the angle of incidence of semiconductor laser light striking the diffraction grating 58, a modulation in the oscillation frequency of semiconductor laser light can be performed. The reason for the provision of a polarizer 59 between the collimating lens 51 and the focusing lens 52 will be discussed later.
In order to allow the semiconductor laser 50 and the output mirror 57 to act as an external resonant cavity, an emitting portion of the semiconductor laser 50 carries thereon a coating antireflective (AR) to semiconductor laser light and the output mirror 57 carries thereon a coating high-reflective (HR) to semiconductor laser light but antireflective to second-harmonic light. Each end portion of the waveguide 54 carries thereon an AR coating to reduce optical loss.
To accomplish high fundamental-to-second-harmonic conversion performance by means of the waveguide 54 formed in a substrate of LiNbO.sub.3, LiTaO.sub.3, or KTP, it is necessary to introduce semiconductor laser light with TM (Transverse Magnetic) mode (TM wave: an electromagnetic wave whose polarization direction is in a direction of the z-axis) oscillation, into the waveguide 54.
In the above-described conventional lasing system, semiconductor laser light emitted from the semiconductor laser 50 oscillates in a TE (Transverse Electric) mode (TE wave: an electromagnetic wave whose direction of polarization is within a layer surface). Because of this, when placing the semiconductor laser 50 and the LiTaO.sub.3 substrate 53 on respective planes parallelling with each other (see FIG. 15a), the polarization direction of semiconductor laser light crosses orthogonally with respect to the polarization direction of second-harmonic light. As a result, no phase matching is possible (see FIG. 15b) and no conversion from semiconductor laser light to second harmonic light occurs. If, as shown in FIG. 16a, the semiconductor laser 50 and the LiTaO.sub.3 substrate 53 are placed on respective planes orthogonally crossing with each other, this aligns the polarization direction of semiconductor laser light with the polarization direction of second-harmonic light (see FIG. 16b). However, due to the fact that the major axis of semiconductor laser light having an ellipse form and the minor axis of second-harmonic light having an ellipse form are in alignment with each other, the drop in coupling efficiency occurs and the conversion efficiency falls.
Accordingly, as a solution to the above problem, a technique has been used. In this technique, semiconductor laser light emitted from the semiconductor laser 50 undergoes a mode conversion from TE mode to TM mode and is coupled to the incident portion of the waveguide 54, to adjust the beam form of semiconductor laser light to the population form of second-harmonic light. One way of subjecting semiconductor laser light to a TE-to-TM conversion is to arrange the polarizer 59 between the collimating lens 51 and the focusing lens 52 (see FIG. 13). Another way is to insert between the collimating lens 51 and the focusing lens 52 a half wave plate 60 capable of producing a phase difference of a half wave length.
However, these two TE-to-TM conversion techniques suffer from respective problems. The first way that makes use of the polarizer 59 produces the problems that the oscillation threshold current of the semiconductor laser 50 is substantially increased and the differential efficiency is decreased because semiconductor laser light oscillates with considerable loss of TE mode components.
The second way that makes use of the half wave plate 60 produces the problems that, since the half wave plate 60 is placed within the external resonant cavity causing transmission loss, the oscillation threshold current of the semiconductor laser 50 is increased and the differential efficiency is decreased.
Both the increase in the oscillation threshold current and the drop in the differential efficiency result in increasing the semiconductor laser operating current.